Subcellular components of the finch mouse, chicken, and cat ventricular and atrial cells will be quantitated by stereologic techniques. The data from each species will be compared with the other species. These animals were chosen because of their similarities and differences in heart rate and morphology. Two, the mouse and finch, have very rapid heart rates, but differ in ventricular fiber size and in subcellular architecture, e.g., distribution of sarcoplasmic rates and will be compared with the more rapidly beating mammalian and avian hearts. These data will also be compared with calcium kinetic data obtained as part of a separate project.